1Field of the Invention
This invention relates to combiners for r.f. power amplifiers.
2Background Information
Higher power r.f. power amplifiers for transmitters operating at higher frequencies (say HF and upwards) are tending to be made of a number of identical lower power r.f. power amplifier modules, typically solid state, of e.g. 250 to 1000 watt connected in parallel in typical numbers of up to 40 or more.
It has been proposed (FIG. 1) to couple sequentially a number (e.g. 20) of power amplifier modules 1.sup.1 . . . 1.sup.20 on to a transmission line 2 via directional couplers 3, to produce a combined output on load 4. Each module receives the same drive input, and the outputs are combined in parallel.
A directional coupler (FIG. 2) has four ports and the property that, assuming all ports are properly matched, a proportion of the input signal at port A appears across a load connected to port C while none appears across a balance load connected to port D (since two equal signals add at port C and subtract at port D). The remainder of the input appears across the output load. The coupling can be via discrete components or by electromagnetic coupling of e.g. two parallel conductors in an outer sheath spaced apart from each other by a certain distance over a certain length e.g. one quarter wavelength. The effective isolation of port D from the input appearing at port A enables the directional coupler to be used to combine two inputs e.g. one appearing at port A and one appearing at port D. An input appearing at port D in place of the matched load would be split between port C and port B.
In FIG. 1 the couplers are used in this way to combine two appropriately phased signals. Thus, as shown in greater detail in FIG. 3 (the coupler being shown in its symbolic form, and the phase shift across each of the paths AB, CD being 90.degree.) a first signal entering at port A, nominal phase 0.degree. is divided into one part leaving port B at nominal phase -90.degree. and another part leaving port C nominal phase 0.degree.. A second signal entering at port D, nominal phase -90.degree. is divided into one part appearing at port C, nominal phase -180.degree. and the remainder appearing at port B, nominal phase -90.degree.. It will be apparent that an appropriate degree of coupling between the paths AB, CD together with appropriate relative voltage amplitudes of the alternating signals entering at ports A and D results in cancellation of the signal appearing at port C connected to the balance load and summation of the signals leaving port B.